In 3rd Generation Partnership Project Radio Access Network Long Term Evolution (3GPP-LTE (hereinafter referred to as LTE)), Orthogonal Frequency Division Multiple Access (OFDMA) is adopted as a downlink communication scheme, and Single Carrier Frequency Division Multiple Access (SC-FDMA) is adopted as an uplink communication scheme (e.g., see NPL-1, NPL-2, and NPL-3).
In LTE, a base station apparatus for radio communications (hereinafter abbreviated as “base station”) performs communications by allocating a resource block (RB) in a system band to a terminal apparatus for radio communications (hereinafter abbreviated as “terminal”) for every time unit called “subframe.” The base station also transmits allocation control information (i.e., L1/L2 control information) for the notification of the result of resource allocation of downlink data and uplink data to the terminal. The allocation control information is transmitted to the terminal through a downlink control channel such as a Physical Downlink Control Channel (PDCCH). A resource region to which a PDCCH is to be mapped is specified. As shown in FIG. 1, a PDCCH covers the entire system bandwidth in the frequency-domain and the region occupied by the PDCCH in the time-domain varies between a leading first OFDM symbol and a third OFDM symbol in a single subframe. A signal indicating a range of OFDM symbols occupied by a PDCCH in the time-domain direction is transmitted through a Physical Control Format Indicator Channel (PCFICH).
Each PDCCH also occupies a resource composed of one or more consecutive control channel elements (CCEs). In a PDCCH, one CCE consists of 36 resource elements (RE). In LTE, the number of CCEs occupied by a PDCCH (CCE aggregation level, or simply aggregation level) is selected from 1, 2, 4, and 8 depending on the number of bits of allocation control information or the condition of a propagation path of a terminal. In LTE a frequency band having a system bandwidth of up to 20 MHz is supported.
Allocation control information transmitted from a base station is referred to as downlink control information (DCI). If a base station allocates a plurality of terminals to one subframe, the base station transmits a plurality of items of DCI simultaneously. In this case, in order to identify a terminal to which each item of DCI is transmitted, the base station transmits the DCI with CRC bits included therein, the bits being masked (or scrambled) with a terminal ID of the transmission destination terminal. Then, the terminal performs demasking (or descrambling) on the CRC bits of a plurality of items of possible DCI directed to the terminal with its own ID, thereby blind-decoding a PDCCH to detect the DCI directed to the terminal.
DCI also includes resource information allocated to a terminal by a base station (resource allocation information) and a modulation and channel coding scheme (MCS). Furthermore, DCI has a plurality of formats for uplink, downlink Multiple Input Multiple Output (MIMO) transmission, and downlink non-consecutive band allocation. A terminal needs to receive both downlink allocation control information (i.e., allocation control information about a downlink) and uplink allocation control information (i.e., allocation control information about an uplink) which have a plurality of formats.
For example, for the downlink allocation control information, formats of a plurality of sizes are defined depending on a method for controlling a transmission antenna of a base station and a method for allocating a resource. Among the formats, a downlink allocation control information format for consecutive band allocation (hereinafter simply referred to as “downlink allocation control information”) and an uplink allocation control information format for consecutive band allocation (hereinafter simply referred to as “uplink allocation control information”) have the same size. These formats (i.e., DCI formats) include type information (for example, a one-bit flag) indicating the type of allocation control information (downlink allocation control information or uplink allocation control information). Thus, even if DCI indicating downlink allocation control information and DCI indicating uplink allocation control information have the same size, a terminal can determine whether specific DCI indicates downlink allocation control information or uplink allocation control information by checking type information included in allocation control information.
The DCI format in which uplink allocation control information for consecutive band allocation is transmitted is referred to as “DCI format 0” (hereinafter referred to as “DCI 0”), and the DCI format in which downlink allocation control information for consecutive band allocation is transmitted is referred to as “DCI format 1A” (hereinafter referred to as “DCI 1A”). Since DCI 0 and DCI 1A are of the same size and distinguishable from each other by referring to type information as described above, hereinafter, DCI 0 and DCI 1A will be collectively referred to as DCI 0/1 A.
In addition to these DCI formats, there are other formats for downlink, such as DCI format 1 used for non-consecutive band allocation (hereinafter referred to as DCI 1) and DCI formats 2 and 2A used for allocating spatial multiplexing MIMO transmission (hereinafter referred to as DCI 2 and 2A). DCI 1, DCI 2, and DCI 2A are formats that are dependent on a downlink transmission mode of a terminal (non-consecutive band allocation or spatial multiplexing MIMO transmission) and configured for each terminal. In contrast, DCI 0/1A is a format that is independent of the transmission mode and can be used for a terminal having any transmission mode, i.e., a format commonly used for every terminal. If DCI 0/1A is used, single-antenna transmission or a transmit diversity scheme is used as a default transmission mode.
Also, for the purpose of reducing the number of blind decoding operations to reduce a circuit scale of a terminal, a method for limiting CCEs targeted for blind decoding for each terminal has been under study. This method limits a CCE region that may be targeted for blind decoding by each terminal (hereinafter referred to as “search space”). As used herein, a CCE region unit allocated to each terminal (i.e., corresponding to a unit for blind decoding) is referred to as “downlink control information allocation region candidate (i.e., DCI allocation region candidate)” or “unit region candidate targeted for decoding.”
In LTE, a search space is configured for each terminal at random. The number of CCEs that form a search space is defined per CCE aggregation level of a PDCCH. For example, as shown in FIG. 2, the numbers of CCEs forming search spaces are 6, 12, 8, and 16 for PDCCH CCE aggregation levels 1, 2, 4, and 8, respectively. In this case, the numbers of unit region candidates targeted for decoding are 6 (=6/1), 6 (=12/2), 2 (=8/4), and 2 (=16/8)for PDCCH CCE aggregation levels 1, 2, 4, and 8, respectively (see FIG. 3). In other words, the total number of unit region candidates targeted for decoding is limited to 16. Thus, since each terminal may perform blind-decoding only on a group of unit region candidates targeted for decoding in a search space allocated to the terminal, the number of blind decoding operations can be reduced. A search space in each terminal is configured using a terminal ID of each terminal and a hash function for randomization. A terminal-specific CCE region is referred to as “UE specific search space (UE-SS)”.
The PDCCH also includes control information for data allocation, the information being common to a plurality of terminals and notified to the plurality of terminals simultaneously (for example, allocation information about downlink broadcast signals and allocation information about signals for paging) (hereinafter referred to as “control information for a shared channel”). To transmit the control information for a shared channel, a CCE region common to all the terminals that are to receive downlink broadcast signals (hereinafter referred to as “common search space: C-SS”) is used for the PDCCH. A C-SS includes just six unit region candidates targeted for decoding in total, namely, 4 (=16/4) and 2 (=16/8) candidates for CCE aggregation levels 4 and 8, respectively (see FIG. 3).
In a UE-SS, the terminal performs blind-decoding for the DCI formats of two sizes, i.e., the DCI format (DCI 0/1A) common to all the terminals and the DCI format (one of DCI 1, DCI 2 and DCI 2A) dependent on the transmission mode. For example, in a UE-SS, the terminal performs 16 blind-decoding operations for each of the DCI formats of the two sizes as described above. A transmission mode notified by the base station determines for which two sizes of the DCI formats the blind decoding is performed. In contrast, in a C-SS, the terminal performs six blind-decoding operations on each of DCI format 1C, which is a format for shared channel allocation (hereinafter referred to as “DCI 1C”) and DCI 1A, (i.e., 12 blind decoding operations in total) regardless of a notified transmission mode.
DCI 1A used for shared channel allocation and DCI 0/1A used for terminal-specific data allocation have the same size, and terminal IDs are used to distinguish between DCI 1A and DCI 0/1A. Thus, the base station can transmit DCI 0/1A used for terminal-specific data allocation in a C-SS as well without an increase in the number of blind decoding operations to be performed by the terminals.
Also, the standardization of 3GPP LTE-Advanced (hereinafter referred to as LTE-A), which provides a data transfer rate higher than that of LTE, has been started. In LTE-A, in order to achieve a downlink transfer rate of up to 1 Gbps and an uplink transfer rate of up to 500 Mbps, base stations and terminals (hereinafter referred to as LTE-A terminals) capable of communicating at a wideband frequency of 40 MHz or higher will be introduced. An LTE-A system is also required to support terminals designed for an LTE system (hereinafter referred to as LTE terminals) in the system in addition to LTE-A terminals.
In LTE-A, a new uplink transmission method will be introduced that uses a non-consecutive band allocation and MIMO. Accordingly, the definitions of new DCI formats (e.g., DCI formats 0A and 0B (hereinafter referred to as DCI 1A and DCI 0B)) (e.g., see NPL-4) are being studied. In other words, DCI 0A and DCI 0B are DCI formats that depend on the uplink transmission mode.
As described, in LTE-A, if a DCI format (any one of DCI 1, DCI 2, and DCI 2A) dependent on the downlink transmission mode, a DCI format dependent on the uplink transmission mode (any one of DCI 0A and DCI 0B), and a DCI format independent of the transmission mode and common to all the terminals (DCI 0/1A) are used in UE-SS, then the terminal performs blind-decoding (monitoring) on DCI of the abovementioned three DCI formats. For example, as described above, since a UE-SS needs 16 blind decoding operations per DCI format, the total number of blind decoding operations in the UE-SS is 48 (=16×3). Accordingly, 60 blind decoding operations in total is needed after adding 12 (=6×2), i.e., the number of blind decoding operations for the two DCI formats in the C-SS.
Additionally, in LTE-A, to achieve an increased coverage, the introduction of radio communication relay apparatus (hereinafter referred to as “relay station” or “Relay Node” (RN)) has been specified (see FIG. 4). Accordingly, the standardization of downlink control channels from base stations to relay stations (hereinafter referred to as “R-PDCCH”) is under way (e.g., see NPL-5, NPL-6, NFL-7, and NPL-8). At present, the following matters are being studied in relation to the R-PDCCH. FIG. 5 illustrates an example of an R-PDCCH region.                (1) A mapping start position in the time-domain of an R-PDCCH is fixed at the fourth OFDM symbol from the beginning of a subframe, and thus does not depend on the rate at which a PDCCH occupies OFDM symbols in the time-domain.        (2) As a mapping method in the frequency-domain of an R-PDCCH, two disposing methods, “localized” and “distributed” are supported.        (3) As reference signals for demodulation, Common Reference Signal (CRS) and Demodulation Reference Signal (DM-RS) are supported. The base station notifies the relay station as to which reference signal is used.        (4) Each R-PDCCH occupies a resource composed of one or more consecutive Relay-Control Channel Elements (R-CCEs). The number of REs forming one R-CCE varies for each slot, or for each reference signal location. Specifically, in slot 0, a R-CCE is defined as a resource region having, in the time direction, a range of from the third OFDM symbol to the end of slot 0, and having, in the frequency direction, a range of 1 RB's width (excluding, however, the region onto which the reference signal is mapped). In addition, in slot 1, a R-CCE is defined as a resource region having, in the time direction, a range of from the beginning of slot 1 to the end of slot 1, and having, in the frequency direction, a range of 1 RB's width (excluding, however, the region onto which the reference signal is mapped). However, proposals have also been made to divide the above-mentioned resource region into two in slot 1, and to have each be one R-CCE.        